fix(Xapi#_assertHealthyVdiChain): behave when VDI is undefined
fix(Xapi#_assertHealthyVdiChain): behave when VDI is undefined

Fixes vatesfr/xo-web#2189

vatesfr/xo-web,vatesfr/xo-web,lmcro/xo-web,lmcro/xo-web,lmcro/xo-web